February used to be the best month of the year to go, because of the low temperatures (climbing up in the blistering sun is not that pleasant) and it hardly rained in january and february. Since a few years it rains also in January and February, but still February is considered the best month to go.
I went once, and it was in April, temperature at night probably 5 or 6 degrees Celsius) (at the last stop before the top (la comparticion)). Now and in february it should be less. During the day it goes up fast. Anyway, you'd bring a sweater for the night, but shouldn't wear it when going up the last part, from comparticion to the top (often early in the morning), it's cold at first but the hike will make you warm fast enough and once the sun is out it warms up fast.
On a side note: it was the best trip I made in this country.